The present invention relates to a thermoelectric element, and particularly to effecting electrical connection and mechanical engagement with a thermoelectric elemental body formed of thermoelectric semiconductors.
Compared with a metallic thermocouple, a thermoelectric element employing semiconductors of iron silicide (FeSi.sub.2) produces a higher output voltage and has a relatively greater internal resistance, as shown in Table 1 below.
TABLE 1 ______________________________________ Thermoelectric Thermocouple element ______________________________________ Output voltage (mV) up to 20 up to 350 Internal approx. 10 approx. 1000 resistance (m.OMEGA.) ______________________________________
In the case of a thermocouple, it is eventually necessary, due to the low output voltage and the small internal resistance thereof, to reduce the contact resistance in transmitting output electric energy to an external apparatus while giving adequate consideration to the electrical junction, including the connection of lead wires.
FIG. 1 shows an equivalent circuit with the output voltage V0 of a thermocouple or thermoelectric element, its internal resistance Rin and the resistance Rc in the connection of a load Rl. The voltage VRl applied across the load is expressed as ##EQU1## Assuming now that a metallic thermocouple has circuit constants of Rin=10 m.OMEGA., Rl=10 m.OMEGA., and Rc=5 m.OMEGA., ##EQU2## Thus, the voltage VRl actually applied across the load is decreased to one-third of the output voltage V0; as a result, insufficient electric energy is transmitted to an external device.
In the case of a thermoelectric element using semiconductors of iron silicide or the like and having circuit constants of Rin=1 .OMEGA., R1=1 .OMEGA., and Rc=5 m.OMEGA., the following results: ##EQU3## Consequently, ample electric energy results from the thermoelectric element without any substantial adverse influence of the resistance Rc.
It will be evident from the above that, as compared with a metallic thermocouple, a thermoelectric element employing semiconductors of iron silicide or the like has a greater degree of freedom with regard to the connection of lead wires and the like.
However, there exist some difficulties in using such a thermoelectric element, particularly with regard to mechanical engagement therewith and lead wire connection thereto.
In each of a thermocouple and a thermoelectric element, the output voltage V0 generated is substantially proportional to the difference between the hot-junction temperature THi and the cold-junction temperature Tc. This relationship can be expressed as follows with respect to the Seebeck coefficent .alpha. of the thermoelectric material used and the temperature difference .DELTA.T: EQU V0.varies..alpha..DELTA.T
Where .DELTA.T=THi-Tc
To attain a high output voltage V0, it is thus necessary to minimize the cold-junction temperature Tc (to maximize the temperature difference .DELTA.T between Tc and THi). Accordingly, in the case of a thermoelectric element, its electrode structure needs to be designed properly with sufficient consideration given to the cooling effect. With regard to such relationship, Table 2 below lists a comparison between a thermoelectric element and a thermocouple.
TABLE 2 ______________________________________ Material Thermal Material Exterior Conductivity resistance shape Cooling ______________________________________ Thermo- Metal Small Thin and Adequate couple High long with lead wires Thermo- Semiconductor Great Thick and Cooling electric Low short effect element influential ______________________________________
It will be understood from the above that, as compared with a thermocouple, satisfactory cooling at the cold-end portion should be executed in a thermoelectric element where electric energy is obtained from the transfer of a great amount of heat.
The present invention has been accomplished in view of the above circumstances and its object resides in providing an improved thermoelectric element wherein connecting members such as lead tabs are attached securely to the cold-end portion of a thermoelectric elemental body consisting of a p-type semiconductor and an n-type semiconductor joined with each other to form a pn junction therebetween so that both electrical connection and mechanical engagement are achievable simultaneously to offer convenience for mounting and replacement. In particular, a mechanical support member is anchored so as to surround the cold-end portion, hence achieving both electrical connection and mechanical engagement with certainty while realizing efficient cooling and thermal radiation simultaneously to offer convenience for mounting and replacement of the element.